Modem wireless networks commonly employ CDMA techniques to communicate information between a mobile terminal and a base station. Modulating information using CDMA techniques provides an advantage over other modulation methods because CDMA enables multiple base stations to simultaneously use the same channel space to communicate information. Thus, CDMA permits channel overlap between base stations, which has a number of significant advantages in wireless communication systems, including the reduction of interference between mobile terminals and base stations, the exploitation of wireless network multipath components, and the simultaneous modulation and demodulation of information on multiple channels with multiple base stations.
Soft handoff is one method that uses these advantages to reduce error and increase quality of service for wireless CDMA networks. Soft handoff is a steady-state condition wherein a mobile terminal simultaneously communicates identical information with a plurality of base stations. Soft handoff increases transmission and reception diversity at the mobile terminal, thereby increasing information capacity and quality of service while reducing the requisite signal to noise power ratio necessary to reliably communicate information. Soft handoff typically exists throughout a mobile terminal's network connection; nonetheless, the plurality of base stations that communicate with the mobile terminal may change as the mobile terminal physically changes location, thereby requiring the mobile terminal to switch the base stations with which it communicates.
In order to implement soft handoff within a wireless CDMA network, a mobile terminal and the plurality of base stations it communicates with must perform certain functions in order to maintain the plurality of mobile terminal-base station network connections. First, the mobile terminal must receive multiple base station transmissions on the forward link from the base station to the mobile terminal, and then combine these transmissions to aggregate the information sent by the plurality of base stations. This aggregation reduces the information error rate and increases the quality of service for the mobile terminal. Thus, the base stations must use identical CDMA symbols to modulate information and synchronize their transmissions to the mobile terminal on the forward link for the mobile terminal to accurately aggregate and demodulate the redundant transmissions received from the base stations.
In addition, the plurality of base stations receive multiple mobile terminal transmissions on the reverse link from the mobile terminal to each base station. A mobile switching center aggregates or selects appropriate transmissions from the multiple transmissions received by the base stations in order to reduce the error rate and maintain a sufficient quality of service. Thus, the mobile terminal must also synchronously transmit information modulated with identical CDMA symbols to the plurality of base stations for the mobile switching center to aggregate or select CDMA information received from the mobile terminal.
Wireless CDMA voice networks are one common example of a wireless CDMA network that employs soft handoff. In a wireless CDMA voice network, a plurality of base stations are networked together through a common mobile switching center, which connects the base station network to the public telephone network. The base stations communicate with mobile terminals using layer 2 Radio link Protocol (RLP) frames, which include CDMA-modulated information communicated between the base stations and mobile terminals.
For the forward link, the plurality of base stations synchronously transmit identical voice information that has been modulated using identical CDMA modulation symbols. This identical, synchronous, CDMA modulated voice information is received by the mobile terminal, which aggregates the received information and modulation symbols to combine the information received and demodulate the voice information. Even though the received information will vary in power, delay, and quality due to variances within the plurality of base station channels, aggregation enables the mobile terminal to recover the voice information and provide a greater quality of service.
For the reverse link, the mobile receiver modulates voice information using CDMA coding symbols and transmits the CDMA-modulated information to the plurality of base stations simultaneously. The base stations receive the CDMA-modulated information and forward the information to a mobile service center, which selects or aggregates the information received to maximize the quality of service and overcome transmission errors associated with information received at any single base station. The mobile service center then demodulates the CDMA-modulated information to recover the voice information transmitted by the mobile terminal.
A number of features associated with the wireless CDMA voice networks make it relatively easy to implement soft handoff of mobile terminals. First, voice information requires a relatively small bandwidth, thereby permitting the mobile terminal and mobile switching center to employ simple combination and interpolation techniques to demodulate and recover voice information. Second, the CDMA-modulated voice information communicated between the plurality of base stations and the mobile terminal is identical, and thus the information received at either the mobile terminal or the mobile switching center can be combined and selected using relatively simple techniques.
In contrast, the features of wireless CDMA IP networks are markedly different from wireless CDMA voice networks, and make employment of simple aggregation, interpolation, and selection techniques virtually impossible. First, IP information can have a large and variable bandwidth that varies with the mobile terminal, IP application communicating information, and type of information being communicated. Second, wireless IP information communicated between each base station and mobile terminal is not identical because although the message content transmitted is identical, the IP header information for each message varies according to the IP subnet address of the base station communicating with the mobile terminal.
Modern wireless IP architectures use IP based protocols to communicate messages in data packets between mobile terminals and base stations. In these architectures, IP base stations are connected to a wireless IP backbone network through edge routers, which interface the base stations with the backbone network. Each IP base station performs dual functions as both an RF front end for base station-mobile terminal communications, and as a real-time router for IP data packets communicated between the mobile terminals and the wireless IP backbone network. The wireless IP backbone network further includes an IP-based control plan to route messages to and from the base station network that communicates with mobile terminals.
Variations of wireless IP architectures include architectures wherein each base station's coverage area defines an IP subnet. In these “all IP” architectures, each base station has its own IP subnet address, and each base station and mobile terminal requires layer 3 mobility management techniques to communicate messages between the base station and mobile terminal as the mobile terminal crosses a cell boundary. Thus, a mobile terminal may require an IP subnet address change when it crosses from one cell site to another using Mobile IP, DHCP, DRCP, or other layer 3 mobility management techniques.
Supporting soft handoff of mobile terminals in “all IP” architectures is difficult because each base station includes its own IP subnet address. As a result, data packets communicated between the plurality of base stations and mobile terminal in a soft handoff situation are not identical due to their different header information, which is attributable to the different IP subnet addresses for the base stations. Thus, the IP information communicated between the plurality of base stations and mobile terminal cannot be aggregated according to prior art methods for voice data wherein the voice information communicated between each base station and mobile terminal is identical.